In many industrial filtration processes it is desirable that a filter cake is subjected to a "washing" operation in order to remove traces of the original mother liquor or filtrate. For this purpose horizontal belt filters are often used to great advantage, since their horizontal surface presents an ideal configuration to effect "plug flow" washing. However, in some cases additional washing is desirable. Also, plug flow washing cannot be carried out efficiently in those cases where the residual mother liquor is held inside the particles which make up the cake, as may be the case for instance with fibrous material. In such cases a prolonged leaching operation is required, which in most cases is more dependent on contact time rather than on the quantity of liquor used. For this reason filter cake of this nature is normally discharged as a semi-washed cake and then subjected to a separate leaching operation, after which the leached slurry is passed over a second filter for removal of the leachate.